Inductive power transfer (IPT) or wireless power transfer (WPT) systems have received much attention recently from both the research community and the industrial community. Recent research efforts have focused on several major applications, including wireless charging of consumer electronics, electric vehicles, mobile robots, and medical implants. WPT systems are discussed in, for example, Special Issue: Wireless Power Transmission, Technology & Applications, Proceedings of the IEEE, June 2013, Vol. 101, No. 6.
Existing WPT applications require knowledge of the mutual coupling (k) or mutual inductance (M) between the transmitter (Tx) coil and the receiver (Rx) coil in order to control the power flow from the Tx side to the Rx side. For example, for wireless charging of electric vehicles and portable consumer electronics, the relative positions of the Tx coil and the Rx coil typically cannot be precisely fixed. The nature of a WPT system requires the receiver circuit to be physically separated from the transmitter circuit. In order to have good power control in the receiver circuit, some form of feedback control is required. As is typically used in the art, primary control refers to the use of the transmitter circuit to control the power flow in the receiver circuit. Because the Tx and Rx circuits are physically separated from each other, primary control requires a channel to obtain information on the receiver side for feedback control.
In related art devices, primary-control methods for WPT systems can be classified into the following groups: (i) use of primary side control that requires wireless communication systems to feed information obtained from the receiver circuit back to the transmitter circuit for closed-loop control; or (ii) use of primary (transmitter) control that requires information of the mutual coupling (k) or mutual inductance (M) between the Tx and Rx coils.
Examples of the first type of primary-control method include those of Si et al. (Proc. 2nd IEEE Conf. Ind. Electron. Appl., 2007, pp. 235-239), Malpas et al. (IEEE Trans. Biomed. Circuits Syst., vol. 2, no. 1, pp. 22-29, March 2008), Li et al. (International Power Electronics Conference, 2010, pp: 1050-1055), Kim et al. (Electron. Lett., Vol. 48, No. 8, pp. 452-454, 12 Apr. 2012), and Miller et al. (IEEE Journal of Emerging and Selected Topics in Power Electronics, Vol. 3, No. 1, March 2015, pp: 147-162). FIG. 1 shows a circuit schematic of primary control of a wireless power transfer system using a wireless communication system for feedback purposes (Miller et al., supra.).
Examples of the second type of primary-control method include those of Thrimawithana et al. (IEEE International Conference on Industrial Technology (ICIT), 2010, 14-17 Mar. 2010, pp: 661-666), Zaheer et al. (IEEE-EMBS International Conference on Biomedical and Health Infomatics (BHI 2012), Hong King and Shenzhen, China, 2-7 Jan. 2012, pp: 174-179), Hui et al. U.S. Patent Application Ser. No. 61/862,627, Aug. 6, 2013), Trivino-Cabrera et al. (IEEE International Electric Vehicle Conference (IEVC) 2014, Florence, Italy, 17-19 Dec. 2014, pp: 1-5), and Yin et al. (IEEE Transactions on Power Electronics, Vol. 30, Issue 3, 2015, pp: 1657-1667). In general, such systems either use a pre-determined value of M or estimate the value of M so that the output voltage can be calculated from information available on the transmitter (primary) side.
WPT systems can use a decoupling switch in the receiver circuit. Such decoupling switches have typically been used to decouple a specific load from the transmitter system.
FIG. 2A shows a circuit schematic of a typical circuit that includes a shunt decoupling switch S. It includes a transmitter circuit driving a transmitter (primary) winding with self-inductance L1, which is magnetically coupled to the receiver (secondary) winding L2. The secondary circuit includes a parallel LC resonant tank including L2 and C2. The shunt decoupling switch can also be applied to a secondary circuit with a series resonant LC circuit formed by the series-connected L2 and C2 shown in FIG. 2B. Such a series resonant compensated circuit with a shunt decoupling switch has been reported by Boys et al. (U.S. Pat. No. 7,279,850). Based on circuit theory, the equivalent impedance of the load in the receiver (secondary) circuit can be reflected on the primary side. For the example shown in FIG. 2B, the decoupling switch is a “shunt” switch because it can cause a “short-circuit” to the output of the diode rectifier in the receiver (secondary) circuit. When this shunt switch is closed, the secondary current is shorted to the ground of the secondary circuit and the load is decoupled (i.e., isolated) from the power flow from the primary circuit by the diode D, which is in a blocking state. Thus, the reflected impedance of the load RL to the primary side is very large when the shunt decoupling switch is closed.
Chan et al. (IEEE Transactions on Circuits and Systems—I, Vol. 59, No. 8, August, 2012, pp: 1805-1814) described another type of decoupling switch. The switch is of a “series” type incorporated in the charging protection circuit (FIG. 3A). This “series” switch Ms is used to protect the battery from over-voltage and over-current conditions (FIG. 3B). Under normal conditions, this series switch Ms is turned on, linking the battery load to the rectified direct current (DC) voltage Vs in the secondary circuit. When the battery voltage Vb exceeds a certain upper threshold voltage, Ms will be turned off through the control of the “voltage protection circuit” in order to avoid an over-voltage condition. Ms will be turned on when Vb falls to a lower threshold voltage level. Ms will also be turned off when the charging current exceeds a certain maximum level through the control of the “current protection circuit” (FIG. 3B).
In both of the “shunt” switch and “series” switch cases, the switching actions (i.e., On and Off states of the decoupling switch) will affect the reflected impedance on the primary side. For example, when the shunt decoupling switch S (see, e.g., FIGS. 2A and 2B) is closed, the output of the rectifier in the secondary circuit is “short-circuited”, and the load is isolated from the secondary circuit. Consequently, the reflected impedance of the load to the primary side will change, resulting in a corresponding change in the current and/or voltage of the primary winding. On the contrary, when the series decoupling switch (see, e.g., FIG. 3B) is opened, the load is isolated from the secondary circuit electrically. The reflected load impedance to the primary side will change, causing a corresponding change in the primary winding current and/or voltage.